Tag: WISE

Of the 793,918 asteroids and trans-Neptunian objects (TNOs) currently catalogued, only 98 are in retrograde orbits around the Sun. That’s just 0.01%.

By “retrograde” we mean that the object orbits the Sun in the opposite sense of all the major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. From a vantage point above the north pole of the Earth, all of the major planets orbit in a counterclockwise direction around the Sun.

But a retrograde object would be seen to orbit in a clockwise direction around the Sun, as is shown in the animation below for Jupiter retrograde co-orbital asteroid 514107 (2015 BZ509), with respect to Jupiter and its two “clouds” of trojan asteroids.

Of these 98 retrograde objects, only 14 have orbits well-enough determined to have received a minor planet number, and only one has yet received an official name (20461 Dioretsa).

Semimajor Axis (a) between…

Number of Retrograde Minor Planets

Mars – Jupiter

3

Jupiter – Saturn*

20

Saturn – Uranus*

15

Uranus – Neptune*

20

TNOs

40

*asteroids between the orbits of Jupiter and Neptune are often referred to as centaurs

At least some of these objects may be captured interstellar objects.

Let’s now take a look at some of these 98 retrograde objects in greater detail.

20461 DioretsaThe first retrograde asteroid to be discovered was 20461 Dioretsa, in 1999. The only named retrograde asteroid to date, Dioretsa is an anadrome of the word “asteroid”. It is a centaur in a highly eccentric orbit (0.90), ranging between the orbits of Mars and Jupiter out to beyond the orbit of Neptune. Objects in cometlike orbits that show no evidence of cometary activity are often referred to as damocloids. Dioretsa is both a centaur and a damocloid. Its orbital inclination (relative to the ecliptic) is 160°, which is a 20° tilt from an anti-ecliptic orbit. It takes nearly 117 years to orbit the Sun once. It is a dark object with a reflectivity only around 3% and is estimated to be about 9 miles across.

2010 EQ169This retrograde asteroid holds the distinction (at least temporarily) of being the most highly-inclined main-belt asteroid (91.6°), relative to the ecliptic plane. It is also the retrograde asteroid with the smallest semimajor axis (2.05 AU) and lowest orbital eccentricity (0.10). Unfortunately, it was discovered after the fact by analyzing past data from the Wide-field Infrared Survey Explorer (WISE) space telescope, and has not been seen since. We have only a three-day arc of 17 astrometric observations of 2010 EQ169 between March 7-9, 2010 from which to determine its orbit. Nominally, 2010 EQ169 orbits the Sun at nearly a right angle to the ecliptic plane once every 2.9 years, between the orbits of Mars and Jupiter. However, our knowledge of its orbit is extremely uncertain, as shown below, and it has been lost. Our only hope will be to back-calculate the positions of future asteroids discovered to these dates to see if it matches the WISE positions.

Element

Value

1σ Uncertainty

Inclination (i)

91.606°

18.177°

Semimajor Axis (a)

2.0518 AU

2.2176

Orbital Eccentricity (e)

0.10153

0.90213

Orbital Period (P)

2.94y

4.765

2013 BL76This retrograde TNO has the largest known semi-major axis of any of the retrograde non-cometary objects: 966.4274 ± 2.2149 AU. In a highly eccentric cometlike orbit (e = 0.99135), its perihelion is in the realm of the centaurs between the orbits of Jupiter and Saturn (8.35 AU), and its aphelion is way out around 1,924 AU. It takes about 30,000 years to orbit the Sun. Its orbit is inclined 98.6° with respect to the ecliptic.

2013 LA2This retrograde centaur is in an orbit closest to the ecliptic plane (i = 175.2°), tilted 4.8° with respect to the ecliptic. It orbits the Sun about once every 21 years between the orbits of Mars and Uranus.

2017 UX51The distinction for this retrograde TNO is that it has the highest orbital eccentricity of any non-cometary solar system object (e = 0.9967). Or is it an old inactive comet? 2017 UX51 orbits the Sun every 7,419 ± 2,883 years as close in as between the orbits of Earth and Mars (perihelion q = 1.24 AU)—classifying it as an Amor object—out to far beyond the orbit of Neptune (aphelion Q = 759.54 ± 196.77 AU). Its orbital inclination is 108.2°.

343158 (2009 HC82)An Apollo asteroid, 343158 is the only known retrograde near-Earth asteroid (NEA), with an orbital inclination of 154.4°. It orbits the Sun every 4.0 years, between 0.49 AU (almost as close in as the aphelion of Mercury) out to 4.57 AU (between the orbits of Mars and Jupiter).

A small number of asteroids are currently in a temporary 1:1 orbital resonance with the Earth in their orbit around the Sun. Unlike the Moon, which is in a stable orbit around the Earth, these much tinier “co-orbital” objects are “just passin’ through.”

3753 Cruithne (1986 TO)
Came relatively close to the Earth each November from 1994 to 2015. This will next happen around 2292.WikiJPLOrrery

85770 (1998 UP1)
Passes close to Venus, too. This next happens in 2115.WikiJPLOrrery

54509 YORP (2000 PH5)
This tiny asteroid, perhaps 492 × 420 × 305 feet across, is a rapid rotator, turning around once every 12m10s. It is named after the YORP effect, as it provided the first observational evidence of that effect speeding up its spin rate. It’s day will be half as long in only 600,000 years, and it may eventually speed up to one rotation every 20 seconds!WikiJPLOrrery

2002 AA29
This near-Earth object has an orbit that is very similar to the Earth’s, and even more circular, though it is inclined a full 10.7° to the ecliptic. This asteroid is a good candidate for an automated sample-return mission and then human exploration because it is relatively close to the Earth and the amount of energy needed to visit 2002 AA29 and return to Earth is relatively small.WikiJPLOrrery

164207 (2004 GU9)
Currently, this asteroid never strays far from Earth, sometime leading it and sometimes following it.WikiJPLOrrery

277810 (2006 FV35)
This asteroid is another good candidate for human exploration.WikiJPLOrrery

2006 RH120
This extremely tiny object (just 7 to 10 feet across) spins more rapidly than any other object on our list: once every 2m45s! It may even be an old rocket booster from the Apollo era, but recent evidence indicates it is a bona fide space rock. It is currently leading the Earth in a very similar orbit.WikiJPLOrrery

2009 BD
We’ve been able to observe orbital changes in this tiny object due to the Sun’s radiation pressure. It is currently trailing the Earth.WikiJPLOrrery

419624 (2010 SO16)
This asteroid was discovered using an infrared space telescope (WISE) and is in an unusually stable orbit that will change little during the next several hundred thousand years. It is currently trailing the Earth.WikiJPLOrrery

2010 TK7
Also discovered using WISE, about 1,000 ft. across. The only known Earth trojan asteroid. It currently orbits the Sun about the L4 Lagrange point (leading the Earth by 60°).WikiJPLOrrery

2013 LX28
This asteroid has the highest orbital inclination (50°) of all the objects on our list.WikiJPLOrrery

2015 SO2
Discovered from Slovenia. Currently leading the Earth.WikiJPLOrrery

469219 (2016 HO3)
Currently, a quasi-satellite of the Earth. Always remains within 38 to 100 lunar distances from the Earth as it orbits the Sun. Leads, then follows, then leads again. Quite a do-si-do!WikiJPLOrrery

Acknowledgements
The orrery videos for each asteroid were generated using the Jet Propulsion Laboratory’s incredible Orbit Diagram Java applet on their Small Body Database Browser web site (https://ssd.jpl.nasa.gov/sbdb.cgi), and captured using the equally incredible ScreenFlow software from Telestream (https://www.telestream.net/screenflow/). Kudos to both organizations!